I have problems with reducing nitrate levels. I have a 20 gallon and 50 gallon tropical tanks. I do water changes every 2 weeks. about 25% water change. I have one live plant in the 20 and 3 in the 50. my ph is 6.6 Ammonia and nitrite levels are 0 but levels remain high..Any suggestions would be great. Now for salt. Some say use salt some say no. I use a lil salt. I figure even fresh water has a lil salt in it. Just would like to finally to know salt or no salt, I use aquarium salt. Thanks

You need to increase your water changes and substrate vacuuming to get the nitrate under control, IMO your water changes need to be at least 50% weekly with substrate vacuum.
Depending on your nitrate level you may want to make smaller daily water changes to slowly lower the nitrate so not to shock and kill the fish and then stay on top of the weekly 50% thereafter to keep the nitrate level in the 5-10ppm range.

Salt, this debate has been going on for years, IMO/E no salt for most species, some species of fish and plants it can even be harmful due to sensitivity to salts, long term use and wrong dosage can also cause resistant pathogens/parasite. I only use salt for short term treatment.

First, salt is suggested to combat nitrite, not nitrate; I've never read that salt will keep nitrates low. And you should never have nitrite above zero in an established aquarium. So the salt is not going to help your situation anyway.

As mentioned by oldfishlady, you need to do more water changes, by which I mean more volume. 50% once a week should be good in non-planted tanks (and one plant in a tank is not "planted" obviously). More plants would (or should) lower the nitrates. But this depends upon your fish load. You don't mention what fish or how many; if the fish stocking is normal (reasonable), a 50% weekly partial water change should keep nitrates below 20 ppm. While most fish can tolerate higher levels, it is best to keep them below 20 ppm; regardless of what level may or may not hurt this or that fish, below 20ppm means a healthier system and that is the aim.

Last comments on salt. I wrote on this only yesterday in another thread, so here is that copied over.

Salt is not something that should go into a freshwater fish aquarium, except perhaps as a treatment for specific issues. I say perhaps because there are some fish that do not tolerate salt and I would never use it for any reason: characins (tetras, pencilfish, hatchetfish), most catfish that occur in soft water including Corydoras, Farlowella, etc., and soft water SE Asian species. Livebearers can tolerate salt better, but it is not necessary or advised in general, even with mollies. These latter do occur in brackish water which is why many recommend salt with mollies, but they are freshwater fish and do just as well without salt.

The issue with salt is internal; it is a bit involved, but as fish take in water via osmosis through their cells (comparable to our drinking water), the salinity as well as the pH and hardness of the water has an effect on their physiology. In one article I read, Laura Muha described it thus: "When pH and/or salinity stray outside the ideal range for any given species, the fishes' bodies must work harder and use more energy to maintain the physiological equilibrium." This adds stress to the fish, and stress affects the immune system and other functions. No authority I have so far read recommends using salt with any freshwater fish except as a specific treatment.

Plants will not grow with salt; I have not experimented to see what level the salt has to reach before it becomes detrimental to plant growth, as I never intend using salt anyway. But there is absolutely no question that using salt will cause our common aquarium plants to die; which is one reason why salt treatment for ich was never recommend in planted tanks.

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian.[unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]